Semiconductor diode lasers were developed in 1962 almost simultaneously by several groups of workers. Since then much development work has evolved in the development and use of light emitting semiconductor diodes.
There are at least three groups of diode lasers and are classified according to structure. Simple diode lasers are called homostructure lasers because they are made of a single semiconductor material. A homostructure diode laser would comprise, for example, n-type and p-type gallium arsenide (GaAs). The recombination of electrons injected from the n-region into the p-region with holes existing in the p-region causes the emission of laser light.
In a single-heterostructure semiconductor laser device on additional layer of aluminum gallium arsenide, (AlGaAs), for example, is added. This type of crystal has had some of the gallium atoms in the gallium arsenide crystal replaced by aluminum atoms. The injected electrons are stopped at the aluminum gallium arsenide layer (junction) resulting in a higher degree of concentration of light emitted.
In a double-heterostructure semiconductor laser device, for example, a layer of GaAs is sandwiched between two layers of AlGaAs. The barriers set up by the heterostructures cause even further confinement of the light emitted.